What it changes

Registering a username on any public Wikimedia wiki automatically reserves that name on all the others; this means different users can no longer register with the same account name on different wikis. Users only need to set and confirm their email address in one account. Changing the password in any wiki changes it in all wikis accordingly. Special:UserLogin now logs the user in to every unified wiki simultaneously, noting that navigating away from the login page before it is fully loaded may result in incomplete login (i.e. users may not log in to all wikis successfully).

Additional wikis will be added to the user's login the first time they are visited. For example, a regular user at Commons and German Wikipedia will not automatically login to English Wikibooks, but if that user visits English Wikibooks once while logged in, they will then login to English Wikibooks every time (to see which wikis you log in to, see Special:CentralAuth).

What it doesn't change

Some things are still local:

User rights are mostly local, which means that administrators will not have administrator access everywhere. Global groups such as global rollback, global sysop, global interface editors and global IP block exemption can be requested at Steward requests/Global permissions.

User preferences are local, although the email address only needs to be set and confirmed in one place. You can continue to have different preferences on different sites. It is possible however to set global preferences if you so wish.[2]

Users can still have differently named accounts on two sites; however, these accounts will not be linked together into one global account.

The global account system is only available for open Wikimedia projects; sites which run on the MediaWiki software but are not operated by the Foundation will continue to have separate account systems, even if they installed CentralAuth extension, which is responsible for the unified login system.

Conflict resolution

The system will automatically merge identically named accounts if they have the same authenticated email address, or the user can correctly provide their password.

Because registration has for a long time been separate for each wiki, there are many user names that belong to different people on different projects. The new system only allows one user per name, so there are some cases where accounts will need to be renamed. This must be done manually by a steward (see Steward requests/Username changes).

I have two or more accounts with different names. Can they be merged into one account?

The existence of the other account does not necessarily stop you from getting the global account. However, you might not be the one with the superior claim to the username if, for example, the other user has more edits or is a member of certain groups such as sysops or bureaucrats. You may have to ask them to rename, or you may have to rename your account.

Will I have autoconfirmed status on other wikis?

No. You will have to wait the appropriate amount of time after first logging into each particular wiki, before getting autoconfirmed status.

Can I merge accounts from restricted-account-creation wikis?

No, this is not currently possible. This is to prevent a user from creating an account on an open wiki and then merging it into a restricted wiki, which would allow the user access to the restricted one.